The present invention relates to a regulating resistor network for regulating voltage or current supplied to a semiconductor circuit within certain limits, a semiconductor device including such a resistor network and a method for fabricating the device.
Hereinafter, a semiconductor device including a conventional regulating (or trimming) resistor will be described with reference to FIGS. 10(a) and 10(b).
As shown in FIG. 10(a), first and second chips B and C are mounted on a printed wiring board A. A field effect transistor (FET) 1 is formed as an amplifier on the first chip B. The drain terminal of the FET 1 is connected to a drain voltage supply terminal 2, while the source terminal thereof is grounded.
A resistance adjustment circuit 4 for regulating a bias voltage applied to the gate terminal 3 of the FET 1 is formed on the second chip C. The resistance adjustment circuit 4 includes a voltage supply terminal V.sub.GG, first and second voltage dividers 5 and 6 and a trimming resistor 7, which are connected in series to each other. In FIG. 10(a), a signal input terminal V.sub.s for applying an input voltage to the gate terminal 3 of the FET 1 is provided on the printed wiring board A.
As shown in FIG. 10(b), the trimming resistor 7 is implemented as a thin-film resistor including a plurality of notches 7a, which have been formed by laser-trimming the thin-film resistor. In such a thin-film resistor, the current path has an increased length. Thus, the resistance value of the thin-film resistor goes on increasing gradually as the current flows over a longer distance, and the resistance value of the trimming resistor 7 is adjustable.
Once the resistance value of the trimming resistor 7 has changed, the amount of a current flowing through the resistance adjustment circuit 4 also changes. As a result, the bias voltage applied to the gate terminal 3 of the FET 1 changes, too. Thus, even if the operating current of the FET 1 has deviated from a predetermined value, the operating current can be regulated at the desired value by adjusting the resistance value of the trimming resistor 7 and thereby changing the bias voltage applied to the gate terminal 3.
Hereinafter, a method of regulating the operating current of the FET 1 by adjusting the resistance value of the trimming resistor 7 will be briefly described.
First, a predetermined voltage is applied to the voltage supply terminal V.sub.GG of the resistance adjustment circuit 4 and a bias voltage is applied to the gate terminal 3 of the FET 1. In such a state, while the operating current of the FET 1 is being monitored, the thin-film trimming resistor 7 is laser-trimmed, thereby gradually increasing the resistance value of the trimming resistor 7.
Thereafter, when the operating current flowing through the FET 1 reaches a predetermined value, the laser trimming of the thin-film resistor is stopped, thereby fixing the current flowing through the resistance adjustment circuit 4 at a constant value.
As described above, to adjust the resistance values of the trimming resistor 7 and the resistance adjustment circuit 4, the thin-film trimming resistor 7 should be laser-trimmed, while the operating current of the FET 1 is being monitored. Thus, it usually takes as long a time as 5 to 7 seconds to complete such fine adjustment.
In addition, since the resistance value of the thin-film trimming resistor 7 should be adjusted by irradiating the trimming resistor 7 with laser beams for such a long time, the trimming resistor 7 and the resistance adjustment circuit 4 both receive a large quantity of heat. Accordingly, if the resistance adjustment circuit 4 and the FET 1 are formed on the same chip, then the laser irradiation on the trimming resistor 7 causes considerable damage to the FET 1.
Thus, in the conventional arrangement, a semiconductor circuit such as the FET 1 and the resistance adjustment circuit 4 should be separately formed on the first and second chips B and C. And then these chips B and C should be mounted on the printed wiring board A. That is to say, the semiconductor circuit and resistance adjustment circuit cannot be integrated on the same chip.